1. Field of the Invention
The present invention relates to a circuit board such as a tape carrier substrate used in a chip-on-film device (COF), and a semiconductor device formed by joining the circuit board and a semiconductor chip.
2. Description of Related Art
As one type of package module using a film substrate, the one employing a COF structure has been known. FIG. 5 is a cross-sectional view showing a part of one example of a COF described in JP 2004-327936 A. The COF includes a semiconductor chip 4 mounted on a tape carrier substrate of a flexible and insulating film substrate 1 and is protected by an encapsulation resin 5. Such a COF mainly is used as a driver for operating a flat panel display.
The tape carrier substrate includes as main components an insulating film substrate 1, conductive wirings 2 formed on a surface of the film substrate 1 and upper surfaces of bumps 3 on the conductive wirings 2. As required, a metal coating 6 is plated on some parts of the conductive wirings 2 and on the bumps 3, while a layer of solder resist 7 as an insulating resin is formed on the other parts of the conductive wirings 2. In general, polyimide is used as a material of the film substrate 1 and copper is used as a material of the conductive wirings 2. The bumps 3 are formed in general by forming the conductive wirings 2 on the film substrate 1 and then plating a metal thereon.
The conductive wirings 2 are connected to the electrode pads 8 on the semiconductor chip 4 via the bumps 3. In a typical connecting method, an encapsulation resin is applied on a region for mounting a semiconductor chip on the tape carrier substrate, and subsequently the electrode pads 8 on the semiconductor chip 4 and the bumps on the tape carrier substrate are placed opposing each other, to which ultrasonic waves, heat and pressure are applied.
FIG. 6 is a plan view showing schematically an example where the electrode pads 8 on the semiconductor chip 4 and the bumps 3 on the tape carrier substrate are placed opposing each other. It should be noted that the film substrate 1 is not shown in the drawing when viewed from the film substrate 1 side, for viewability. As shown in FIG. 6, typically the plural conductive wirings 2 on the region for mounting the semiconductor chip 4 are straight and parallel to each other.
As mentioned above, when mounting the semiconductor chip on the tape carrier substrate, the electrode pads 8 on the semiconductor chip 4 and the bumps 3 on the tape carrier substrate are subjected to ultrasonic waves, heat and pressure. At that time, stresses caused by the ultrasonic waves and the pressure will be concentrated particularly on the corner areas of the semiconductor chip 4, and thus the outermost conductive wirings 2 will be broken easily. FIG. 7 shows a break in a conductive wiring 2 at one corner of the semiconductor chip 4. As shown in FIG. 7, such a break 9 in the conductive wiring 2 will occur most in a bump 3 along the periphery side on the semiconductor chip 4.
In order to cope with the problem, for example, the outermost conductive wirings 2 are made thicker than the remaining conductive wirings 2. In such a case, however, since the bumps 3 are formed by plating a metal after formation of the conductive wirings 2, the bumps 3 on the outermost conductive wirings 2 will become taller than the bumps 3 of the other conductive wirings 2. This causes a level difference among the bumps 3 and results in connection failure. Therefore, this measure is not suitable for mounting the semiconductor chip 4.
Moreover, if the conductive wirings 2 become narrow due to the future trend of making the lead pitch narrower, the above-mentioned break 9 will occur more easily, and it can cause problems.